User blog:Granpa/Weather
Almost all weather occurs within the troposphere which extends from Earths surface up to the base of the stratosphere (about 10 km). Convection ceases at the stratosphere because the heating of ozone in the stratosphere by the sun causes the temperature within the stratosphere to increase with height and this prevents clouds and thunderstorms from rising within it. Earth's weather is primarily driven by rising air in 3 low pressure areas. *The northern hemisphere polar front. **Extratropical cyclones form along the front and move eastward at 12-15 m/s (43-54 km/h) and last 3-5 days. (3600-6000 km) *The Intertropical Convergence Zone. (Equator) **Tropical cyclones (Hurricanes) form here and move westward. *The southern hemisphere polar front. **Extratropical cyclones form along the front and move eastward And to a lesser extent by descending air in 2 high pressure areas: *Northern hemisphere subtropical ridge (horse latitudes) **The Sahara and the Sonoran deserts are here. *Southern hemisphere subtropical ridge (horse latitudes) **The Kalahari and the Atacama deserts are here. Boundary layers From Wikipedia:Planetary boundary layer: In meteorology the planetary boundary layer (PBL) is the lowest 1-2 km of the troposphere and is highly turbulent and vertical mixing is strong. Its temperature usually responds to changes in surface radiative forcing in an hour or less. There are two types of Planetary boundary layer: #Nocturnal planetary boundary layer: turbulence is due to its motion over and its contact with the planetary surface. #Daytime planetary boundary layer: turbulence is also due to thermal instability (thermals, thunderstroms, etc). The atmospheric surface layer is the lowest part of the planetary boundary layer (typically the bottom 10% where the log wind profile is valid). Above the PBL is the free atmosphere, where the wind is approximately parallel to the isobars, while within the PBL the wind is affected by surface drag and turns across the isobars. Atmospheric convection The heating of the ground by the sun produces thermals which become cumulus clouds. Like a hot air balloon, the warm air rises. The rising warm air expands and therefore cools. The Dry Adiabatic Lapse Rate (DALR) is a constant 0.98 °C for every 100 meters it rises. ((°C × 9/5) + 32 = °F). When the rising air reaches the condensation level the moisture within it condenses into droplets and releases the latent heat of condensation (2,260 Joules per gram of water) which causes the warm air to rise even further. The heat released slows the rate of cooling. The Moist Adiabatic Lapse Rate (MALR) varies with temperature but is typically about 0.5 °C for every 100 meters (50 °C per 10 km). *The International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 °C/km from sea level to 11 km. *1 kg of air at 40 °C (104 °F) and 100% humidity carries 47 grams of water.(Vapour pressure of water at 40 °C = 0.0728 bar) *On a summer day, net solar energy received at a lake reaches 15 MJ per square meter per day. The specific heat of dry air at 1 bar is 1 kJ per kg per °C. See Diurnal temperature variation and Planetary boundary layer. If 80% of the energy is used to vaporize water then evaporation = 0.49 cm/day. A moist atmosphere (100% relative humidity) with an environmental lapse rate grater than the Moist Adiabatic Lapse Rate is required to force the atmosphere to be unstable enough for convection. If the warm air is unstable enough then cumulus clouds continue to grow and become thunderstorms (cumulonimbus). *The average thunderstorm produces about 200,000 m3 of rain, but large storms can produce 10 times more rainfall. *Over a 30 minute period a normal thunderstorm releases 1015 Joules of energy (440,000 m3 of water) equivalent to 0.24 megatons of TNT (ten times larger than the bomb over Nagasaki). A storm that lasted 24 hours would release 48 times as much energy (48 x 1015 Joules). *A hurricane (a tropical cyclone) releases 52 x 1018 Joules/day equivalent to 1000 simultaneous non-stop thunderstorms. Convective available potential energy In the image to the right we lift an imaginary parcel of air first along the dry adiabatic from the ground to the Lifted condensation level (LCL). Then along the moist adiabatic to the level of free convection (LFC). Integrating from A to LCL to LFC give Convective inhibition (CIN). CIN is negative buoyancy. It is the opposite of convective available potential energy (CAPE). CIN inhibits convection. Once CIN is overcome a parcel is free to rise through the free convective layer. The free convective layer (FCL) is the layer of potential instability in the troposphere. It is a layer of positive buoyancy (PBE) and is the layer where deep, moist convection (DMC) can occur. It is the layer between the level of free convection (LFC) and the equilibrium level (EL). From Wikipedia:Convective available potential energy: CAPE is calculated by integrating vertically the local buoyancy of a parcel from the level of free convection (LFC) to the equilibrium level (EL). Convective available potential energy (CAPE), is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability. (See Lifted index.) CAPE is measured in joules per kilogram of air. Extremely large values of CAPE can result in explosive thunderstorm development and severe storms. *But if the jet stream is strong enough then severe weather and tornadoes can also develop in an area of low CAPE values. The surprise severe weather event that occurred in Illinois and Indiana on April 20, 2004 is a good example. There was strong low-level wind shear and although overall CAPE was weak (1000 J/kg), there was strong CAPE in the lowest levels (1-3 km) of the troposphere which enabled an outbreak of minisupercells producing large, long-track, intense tornadoes. See here for more information. Downdraft CAPE (DCAPE), estimates the potential strength of evaporatively cooled downdrafts. Dry air aloft has the effect of increasing the instability of the air and also increases the severity of the downdraft. The 100th meridian forms the eastern border of the Texas panhandle with Oklahoma and roughly marks the boundary between the high plains with their dry climate to the west and the humid climates to the east. See HOW DOES DRY AIR ALOFT MAKE STORMS MORE SEVERE?. Dry air from the west, cold air from the north, and moist air from the gulf all converge in Oklahoma. This is what makes severe weather more common in Oklahoma than anywhere else in the US. See the image below. Severe storms :See Tornado emergency and Particularly Dangerous Situation Tropical air is far warmer than air outside the tropics and therefore holds far more moisture and as a result thunderstorms in the tropics are much taller. Nevertheless severe thunderstorms are not common in the tropics because the storms own downdraft shuts off the inflow of warm moist air killing the thunderstorm before it can become severe. Severe thunderstorms (called supercells) are more common outside of the tropics because of the effect of the polar jet stream. Polar jet streams are typically located near the 250 hPa (about 1/4 atmosphere) pressure level, or 7 to 12 km and are strongest in winter. The width of a jet stream is typically a few hundred kilometers or miles and its vertical thickness often less than five km. Speeds over 398 km/h have been measured. Each large meander, or wave, within the jet stream is known as a Rossby wave. The jet stream pushes against the top of the thunderstorm displacing the downdraft so that it can no longer shut off the inflow of warm moist air. As a result severe thunderstorms can continue to feed and grow for many hours whereas normal thunderstorms only last 30 minutes. Updrafts of 300 km/h are possible. (7000 J/kg = 300 km/h). See BWER Supercells contain mesocyclones, an area of organized rotation a few miles up in the atmosphere, usually 1–6 miles (1.6–9.7 kilometres) across. Most intense tornadoes (EF3 to EF5 on the Enhanced Fujita Scale) develop from supercells. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms. *Highest wind speed ever recorded was 480 km/h in the 1999 Bridge Creek–Moore tornado. *The widest tornado on record is the El Reno, Oklahoma tornado of May 31, 2013 with a width of 4.2 km at its peak. *A pressure deficit of 100 millibars was observed when a violent tornado near Manchester, South Dakota on June 24, 2003 passed directly over an in-situ probe that storm chasing researcher Tim Samaras deployed. In less than a minute, the pressure dropped to 850 millibars. 850 mbar is normal air pressure at 1.5 km altitude. 100 millibars corresponds to 325 km/hr = (100 millibars * 1 m3 / 1.225 kg)^0.5 If supercells track to the right or left of the mean wind, they are said to be "right-movers" or "left-movers," respectively. Supercells can sometimes develop two separate updrafts with opposing rotations, which splits the storm into two supercells: one left-mover and one right-mover. Fronts The polar front is a cold front that arises as a result of cold polar air meeting warm subtropical air at the boundary between the polar cell and the Ferrel cell in each hemisphere. A cold front is the leading edge of a cold dense mass of air, replacing (at ground level) a warmer mass of air. Temperature changes across the boundary can exceed 30 °C (54 °F). A narrow line of thunderstorms often forms along the front.﻿ Gust front . Note the Bow shape. See also Super derecho ]] The downdraft from a thunderstorm can create an Outflow boundary called a gust front that acts like a miniature cold front and can spawn new thunderstorms out ahead of the first. Sometimes a Squall line (line of thunderstorms) forms along the gust front. See Mesoscale convective complex. If the storms along the squall line are severe then the squall line becomes a Derecho. This happens on the rare occasions when the jet stream is blowing in such a way that the cold downdraft from the storms falls back behind the front where the cold descending air reinforces the front that produced the storms in the first place. See Rear-inflow jet. Derechos move quickly and produce strong straight line winds. In Spanish derecho means 'straight'. 60% of derechos occur in May, June, and July. See Line echo (multi-bow) wave pattern. According to the National Weather Service (NWS) criterion, a derecho is classified as a band of storms that have winds of at least 25.5 m/s (92 km/h; 50 kn; 57 mph) along the entire span of the storm front, maintained over a time span of at least six hours. All trees regardless of type or size tend to break when wind speed reaches 151 km/h (94 mph). *The derecho and tornado outbreak of April 4–5, 2011 with wind gusts as high as 145 km/h is reportedly one of the most prolific damaging wind events on record. The outbreak was the first in a series of devastating tornado outbreaks in the month of April 2011. *The April 25–28, 2011 Super Outbreak was the largest, costliest and one of the deadliest tornado outbreaks ever recorded. In total, 360 tornadoes were confirmed by NOAA's National Weather Service (NWS) in 21 states from Texas to New York to southern Canada. Tornado tracks in the US (1950-2017): Cyclones Over the ocean large masses of thunderstorms can become cyclones which can grow into hurricanes. From Tropical cyclogenesis: There are six main requirements for tropical cyclogenesis: #water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft) #Rapid cooling with height #High humidity in the lower to middle levels of the troposphere #Enough Coriolis force to sustain a low pressure center #A preexisting low level focus or disturbance #Low vertical wind shear. While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form. Depth of 26 °C isotherm on October 1, 2006: From Wikipedia:Aleutian Low: Cyclones (Hurricanes/Typhoons) that form in the tropical and equatorial regions of the Pacific normally start off by moving toward the west but can veer northward and get caught in the Aleutian Low (the polar front) where they become Extratropical cyclones which move toward the east. This is usually seen in the later summer seasons. *Both the November 2011 Bering Sea cyclone and the November 2014 Bering Sea cyclone were post-tropical cyclones that had dissipated and restrengthened when the systems entered the Aleutian Low region. The storms are remembered as two of the strongest storms to impact the Bering Sea and Aleutian Islands with pressure dropping below 950mb in each system. **The magnitude of the low pressure creates an extreme atmospheric disturbance, which can cause other significant shifts in weather. Following the November 2014 Bering Sea cyclone, a huge cold wave, November 2014 North American cold wave, hit the US bringing record breaking low temperatures to many states. *The record lowest pressure established in the northern hemisphere is the extratropical cyclone of January 10, 1993 between Iceland and Scotland which deepened to a central pressure of 912-915 mb (26.93”-27.02”). Most hurricanes have an eye below 990 millibars. In 2005, hurricane WILMA reached the lowest barometric pressure ever recorded in an Atlantic Basin hurricane: 882 millibars. Hurricanes don't form in the South Atlantic. Tracks of all Tropical cyclones which formed worldwide from 1985 to 2005: Monsoons Air at the equator (Intertropical Convergence Zone) normally travels toward the west but the Indo-Australian monsoon causes so much air to rise over the Maritime Continent (See Tropical Warm Pool) that between Africa and the Maritime Continent the wind reverses direction and equatorial air travels eastward from Africa toward the Maritime Continent. The South American monsoon has a similar effect over the Pacific ocean west of Brazil. See Humboldt Current. :: The South Pacific convergence zone (SPCZ) & South Atlantic convergence zone (SACZ) are Monsoon troughs that branch off the The Intertropical Convergence Zone (ITCZ) at the points where the Indo-Australian monsoon and the South American monsoon occur. *The Inter-Ocean Convergence Zone has traditionaly been called the Congo air boundary. Also called the South Indian Ocean Convergence Zone (SIOCZ) and Oceanic Tropical Convergence Zone (OTCZ). See Asymmetry of the Intertropical Convergence Zone During an El Niño the South American monsoon is unusually strong and the Indo-Australian monsoon is weak. During an La Niña the opposite occurs. See Walker circulation. *During a La Nina, a double ITCZ sometimes forms in the eastern Pacific, with one located north and another south of the Equator, one of which is usually stronger than the other. When this occurs, a narrow ridge of high pressure forms between the two convergence zones. The Madden–Julian oscillation is a traveling pattern that propagates eastward at approximately 4 to 8 m/s (14 to 29 km/h, 9 to 18 mph), through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall. *In the Pacific, strong MJO activity is often observed 6 – 12 months prior to the onset of an El Niño episode, but is virtually absent during the maxima of some El Niño episodes, while MJO activity is typically greater during a La Niña episode. Atmospheric rivers :See also: Great Flood of 1862 and Pineapple Express Extratropical cyclones can become so large that they draw moisture up directly from the tropics in what is called an atmospheric river. (See the image to the right.) Atmospheric rivers are jet streams of moist air that are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than the Earth's largest river, the Amazon.Wikipedia:Atmospheric river *The Amazon discharges more water into the oceans than the next 7 largest rivers. See Zipf's law. (Like many other rivers the Amazon river valley is an Aulacogen.) During atmospheric river events both the Arctic oscillation and the Pacific–North American teleconnection pattern tend to be in the negative phase. *The negative phase of the Arctic oscillation (AO) is associated with cold arctic air (and therefore the polar front) extending further south than normal. *The negative phase of the Pacific–North American teleconnection (PNA) pattern features below-average pressure in the vicinity of Hawaii (a phase of the MJO), and above-average pressure south of the Aleutian Islands. The negative phase tends to be associated with east Pacific cold episodes (La Niña). Seasonal lag From Wikipedia:Seasonal lag: The amount of Sun energy reaching a location on Earth ("insolation", shown in blue) varies through the seasons but the surface temperatures will lag the primary cycle especially over the ocean. (The top 2.5 m of the ocean holds as much heat as the entire atmosphere above it. See Wikipedia:Mixed layer.) The length of seasonal lag varies between different climates, with extremes ranging from as little as 15–20 days (for polar regions in summer and continental interiors) to as much as 2½ months (for oceanic areas). *San Francisco has an exceptionally long seasonal lag but this is due to the normally onshore winds weakening, and sometimes even reversing, in the autumn. See Wikipedia:Devil winds. Maintaining Atmospheric pressure If Earth's atmosphere were only slightly thicker then the air would be warmer and the amount of water vapor in the air would be much greater and lightning would therefore be much more common. The lightning would break apart the air molecules which would be washed down into the sea where they would end up in sediments which get subducted into the Earth. In this way the Earth's average air pressure is maintained at its current level. During most of it's history Earth only had one atmospheric cell that extended from the pole to the equator and as a result Earth was very much warmer. #Hadley cell During an ice_age the Earth only has two cells. Ice ages are probably caused by deforestation caused by megafauna. #Polar cell #Ferrel cell﻿ The Earth's atmosphere currently has 3 cells. #Polar_cell #Ferrel_cell #Hadley_cell﻿ Misc images Blue represents rising air; Red represents sinking air: Atmospheric instability indices :See also: Statistical Analysis of Thunderstorms on the Eastern Tibetan Plateau Based on Modified Thunderstorm Indices Lifted Index From Wikipedia:Atmospheric instability: The lifted index (LI), usually expressed in kelvins, is the temperature difference between the temperature of the environment Te(p) and an air parcel lifted adiabatically Tp(p) at a given pressure height in the troposphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable. Thunderstorms are expected with values below −2, and severe weather is anticipated with values below −6. K Index From Wikipedia:Atmospheric instability: The K index is derived arithmetically: K-index = (850 hPa temperature – 500 hPa temperature) + 850 hPa dew point – 700 hPa dew point depression * The temperature difference between 850 hPa ( above sea level) and 500 hPa ( above sea level) is used to parameterize the vertical temperature lapse rate. * The 850 hPa dew point provides information on the moisture content of the lower atmosphere. * The vertical extent of the moist layer is represented by the difference of the 700 hPa temperature ( above sea level) and 700 hPa dew point. Bulk Richardson Number From Wikipedia:Atmospheric instability: The Bulk Richardson Number (BRN) is a dimensionless number relating vertical stability and vertical wind shear (generally, stability divided by shear). It represents the ratio of thermally-produced turbulence and turbulence generated by vertical shear. Practically, its value determines whether convection is free or forced. High values indicate unstable and/or weakly sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 10 to 45 suggest environmental conditions favorable for supercell development.. Showalter index From Wikipedia:Atmospheric instability: The Showalter index is a dimensionless number computed by taking the temperature at the 850 hPa level which is then taken dry adiabatically up to saturation, then up to the 500 hPa level, which is then subtracted by the observed 500 hPa level temperature. If the value is negative, then the lower portion of the atmosphere is unstable, with thunderstorms expected when the value is below −3. The application of the Showalter index is especially helpful when there is a cool, shallow air mass below 850 hPa that conceals the potential convective lifting. However, the index will underestimate the potential convective lifting if there are cool layers that extend above 850 hPa and it does not consider diurnal radiative changes or moisture below 850 hPa. Snow tires Brakes stop the tires. Tires stop the car. In cold temperatures the rubber of a regular tire stiffens. Winter tires are designed to remain flexible, allowing the tire to grip the road better. (But winter tires can become too soft in summer.) From Wikipedia:Snow tire: Attributes that can distinguish snow tires from summer tires include: * An open, deep tread, whose void ratio between rubber and spaces between the solid rubber is comparatively high. * Additional thin slits (called siping) in the rubber, that provide more biting edges and improve traction on wet or icy surfaces. * Shoulder blocks—specialized tread design at the outside of the tire tread to increase snow contact and friction. * A narrower tire to minimize resistance from the plowing effect of the tire through deeper snow. * Hydrophilic rubber compounds that improve friction on wet surfaces Wet-film conditions on hard-compacted snow or ice require studs or chains. Tsunami :See International Tsunami Information Center , List of historical tsunamis, and Tsunami earthquake These should only be regarded as average (not maximum) figures for regions very close to the epicenter of the earthquake. Actual values vary considerably. Actual values ranging anywhere from twice the average down to half the average are common. Unexpected tsunamis only a few meters tall have been known to kill hundreds of people. ("Wave height" is twice the "wave amplitude".) The preliminary computer generated estimate of earthquake magnitude (probably based on ML) is often too small and upon inspection by professional seismologists quickly gets updated to a larger value (probably based on Mw). Increases of 0.5 magnitude are not uncommon. An increase in magnitude of 0.5 doubles the height of the expected tsunami. If the tsunami wave is funneled into a narrow bay with a progressively decreasing width then the wave gradually becomes narrower but the total energy of the wave remains the same therefore when the bay has become four times narrower then the wave will have become twice as high. See Green's law. Complicating things even further, even small earthquakes can cause underwater landslides which can produce very large tsunamis. In general, do not try to escape by car. After a major earthquake roads may be damaged or clogged with those trying to escape. Your best bet is to get to the top of a hill or the roof of a reinforced concrete structure six stories above sea level. : Then let them flee to the hills. Do not let the one who is on the housetop go down to get any thing out of his house. Neither let the one who is in the field turn back to get his jacket. The 2011 tsunami inundation extended 5 km inland over very flat ground (see the image below). It would take one hour to walk 5 km and half an hour to jog it. The human body can only sprint for about 350 meters. Along the river the inundation extended 10 km inland. (Jogging burns 10 calories a minute) Close to, and directly in front of, the earthquake the first wave is usually the biggest but the further away the wave travels the less certain that becomes. After the 2011 Japan earthquake it was the fourth wave to hit Tahiti (9500 km from Japan) that was the largest and the all clear had already been broadcast when it arrived. See Sequencing of tsunami waves: why the first wave is not always the largest. The energy required to lift a section of water 100 km by 15 km by 7 km meters deep a distance of 10 meters is 10^18 J. See How Japan's 2011 Earthquake Happened (Infographic) From Wikipedia:Tsunami: The velocity of a tsunami is the the square root of the depth of the water multiplied by the acceleration due to gravity (approximately 10 m/s2). For example, if the continental shelf is 150 m deep, the velocity of a tsunami would be the square root of (150 × 10) = √1500 = ~40 m/s, which equates to a speed of ~140 km/h or about 90 mph. When the depth decreases by a factor of sixteen then the waves are four times slower and twice as high. An earthquake with a magnitude 7-7.9 occurs somewhere in the world about 13 times every year. An earthquake with a magnitude 8-8.9 occurs somewhere in the world about 1.3 times every year. An earthquake with a magnitude 9-9.5 occurs somewhere in the world about once every ten years. See Gutenberg–Richter law The magnitude 9.5 1960 Valdivia earthquake was preceded by three foreshocks: :An 8.1 the day before. :A 7.1 that morning. :A 7.8 just 15 minutes before the main earthquake. The 9.0 2011 Tōhoku earthquake and tsunami was preceded by two foreshocks: :A 7.3 two days before :A 6.4 one day before See Spatial organization of foreshocks as a tool to forecast large earthquakes From Wikipedia:Richter magnitude scale: The total energy release of an earthquake closely correlates to its destructive power. A difference in magnitude of 2.0 is equivalent to a factor of 1000 in the energy released. A difference in magnitude of 1.0 is equivalent to a factor of 31.6 in the energy released. Because of various shortcomings of the original "magnitude scale" developed by Charles F. Richter, and later revised and renamed the Local magnitude scale, denoted as "ML" or "ML", most seismological authorities now use other scales, such as the moment magnitude scale (Mw), to report earthquake magnitudes, but much of the news media still refers to these as "Richter" magnitudes. All scales, except M_\text{w} , saturate for large earthquakes, meaning they are based on the amplitudes of waves which have a wavelength shorter than the rupture length of the earthquakes. These short waves (high frequency waves) are too short a yardstick to measure the extent of the event. The resulting effective upper limit of measurement for M_L is about 7 and about 8.5 for M_\text{s} . M_\text{L} is the scale used for the majority of earthquakes reported (tens of thousands) by local and regional seismological observatories. For large earthquakes worldwide, the moment magnitude scale (MMS) is most common, although M_\text{s} is also reported frequently. Tectonic plates can do one of three things: # Slide past one another. # Move away from each other. # Move toward each other with one plate sliding below the other. It is the third type that produces large tsunamis. Areas where this happens are called subduction zones. Subduction zones are colored blue in the image below. Since 1900, all earthquakes greater than magnitude 8.6 (See here and here) have occured at subduction zones. During an earthquake the plates grind past one another creating heat and causing a thin layer of rock along the fault to become molten. See Fault friction References External links *Ventusky *precipitable water *Madden-Julian Oscillation monitoring *Weekly mjo update *National Weather Service *Meteorology for Scientists and Engineers *AAO, AO, NAO, PNA *Near Real-Time Global Precipitation *https://eldoradoweather.com/climate/world-maps/world-snow-ice-cover.html *https://climate.rutgers.edu/snowcover/ *Jet stream *Entire Year of Weather Radar (2018 U.S. Time Lapse) Category:Blog posts